1. Field of the Invention
The invention relates to an input circuit for receiving signals at an input on an integrated circuit, particularly at an input on a DRAM circuit, and for assessing the received signal with respect to a reference voltage.
2. Description of the Related Art
Fast data links between integrated circuits involve the use of terminated data lines in order to avoid signal reflections in an input circuit in the integrated circuit. In addition, the input circuits provide the data lines with a termination voltage around which the signals transmitted on the data line fluctuate, ideally with the same swing. The termination voltage is prescribed by the input circuit and specifies a center voltage around which the signals to be transmitted move.
The termination usually implemented in current DRAM standards in the input circuits is integrated directly in the integrated circuit and cannot be manipulated from the outside. Normally, the termination voltage is generated by a voltage divider made up of two resistors. Process-related fluctuations or temperature fluctuations mean that the termination voltage is not constant, as a result of which the signals received via the input circuit move around a center voltage which cannot be stipulated exactly and is prescribed by the termination circuit.
To assess the received signal, the respective potential level of the received signal is compared with a reference voltage which can be prescribed externally or can be set in the integrated circuit. The reference voltage cannot be aligned for different input circuits, however, but rather is normally provided for all input circuits on a general basis. It is thus conceivable that the termination voltage and the reference voltage will differ from one another, which means that the voltage swings between the high level and the reference voltage and the voltage swing between the low level and the reference voltage will be of different magnitudes. This may result in one of the high or low levels not being able to be identified reliably under all circumstances. Another drawback is that, with cyclic signals having a predetermined duty ratio, any discrepancy between the termination voltage and the reference voltage can result in a change in the duty ratio of the received signal. This is the case particularly for radiofrequency signals, which have a small edge gradient.